Fresh Concrete Pressure Research Articles

Compressive stress-strain response of freshly compressed rubberized concrete: An experimental and theoretical study

Although using crumb tire rubber concrete (CTRC) in constructing structural elements is accompanied by various environmental and economic advantages, it always comes with challenges due to a significant loss in some mechanical properties of this concrete type. To tackle this challenge, a novel technique of compressing fresh concrete was used, and its impacts on improving the mechanical features of CTRC were investigated. For this purpose, concrete specimens containing crumb tire rubber (CTR), as the fine aggregate substitute, were constructed considering the variables of the water-to-cement ratio (w/c), volume percentage of CTR, and the fresh concrete pressure level as a fraction of steel tube (mold) yielding stress. After the application of a specified short-term pressure to the fresh mix inside the steel molds and the time required for the concrete setting, the steel tubes were removed after curing, and the concrete core alone was exposed to an axial compressive load. According to the test results, the mechanical properties, weight loss resulting from compressing the fresh concrete, energy absorption, failure pattern, the concrete microstructure, water absorption, and porosity were evaluated. By applying an initial axial pressure of 25–80 MPa on the mixes containing 0 %, 15 %, and 30 % CTR replacing the fine aggregate, the compressive strength, the ultimate strain, the toughness, and the initial modulus of elasticity experienced 100–140 % increase, 130–420 % increase, 120–1140 % increase, and 66–70 % decrease, respectively, compared with the uncompressed specimen. Finally, a regression analysis was employed to propose prediction models for the compressive capacity, modulus of elasticity, peak strain, ultimate strain, and relative toughness of freshly compressed concrete containing CTR particles; these models show proper agreement with the experimental results.

Pumping lightweight aggregate concrete into high-rise buildings

Lightweight aggregate concrete (LWAC), characterized with low density and high crushing strength to apparent density ratio, is a potential construction material for modern concrete structures. However, experiences in pumping LWAC to high-rise buildings are insufficient and the pumping pressure of fresh concrete may differ from normal weight concrete (NWC). Herein, three groups of shale lightweight aggregate (SLWA) were prepared to evaluate the workability of LWAC before and after pumping up to 102-m construction site. Water content and pressure loss are analyzed to understand the mechanism in slump variation of LWAC. A pumping pressure model is proposed based on friction assumption for horizontal and vertical pumping. The relationship between water content and ambient pressure is simulated. Pressure drop in pumping LWAC is found more than 4 times that in NWC at the same slump level. The main reason for large slump loss and pumping difficulty for LWAC lies in the water transportation process from mixture into porous aggregates.

Modified Artificial Neural Networks and Support Vector Regression to Predict Lateral Pressure Exerted by Fresh Concrete on Formwork

In this study, a modified Artificial Neural Network (ANN) and Support Vector Regression (SVR) with three different optimization algorithms (Genetic, Salp Swarm and Grasshopper) were used to establish an accurate and easy-to-use module to predict the lateral pressure exerted by fresh concrete on formwork based on three main inputs, namely mix proportions (cement content, w/c, coarse aggregates, fine aggregates and admixture agent), casting rate, and height of specimens. The data have been obtained from 30 previously piloted experimental studies (resulted 113 samples). Achieved results for the model including all the input data provide the most excellent prediction of the exerted lateral pressure. Additionally, having different magnitudes of powder volume, aggregate volume and fluid content in the mix exposes different rising and descending in the lateral pressure outcomes. The results indicate that each model has its own advantages and disadvantages; however, the root mean square error values of the SVR models are lower than that of the ANN model. Additionally, the proposed models have been validated and all of them can accurately predict the lateral pressure of fresh concrete on the panel of the formwork.

Preventing plastic shrinkage cracking by monitoring the capillary pressure build-up in self-compacting concrete

Plastic shrinkage cracking is a problem for the service life, aesthetics and durability of concrete members. It occurs when tensile stresses are induced in fresh concrete, through plastic shrinkage that is restrained. Plastic shrinkage is caused by negative capillary pressure build-up, but the capillary pressure of fresh concrete can be controlled to prevent plastic shrinkage. In this paper, the control of capillary pressure to prevent plastic shrinkage cracking using high-capacity capillary pressure sensors is explored. The capillary pressure behaviour of a self-compacting concrete mix design in various evaporation rates was determined. Capillary pressure boundaries for concrete wetting to relieve tensile stresses were identified. The effectiveness of wetting fresh concrete at a predefined capillary pressure boundary was investigated.

Pre-applied mechanical bonded waterproofing membranes: Macroscopic nonwoven behavior during the concreting process

Pre-applied bonded waterproofing membranes are used as positive-side waterproofing membranes on concrete basements. They are installed prior to concrete works and form a bond with the fresh concrete, inter alia, by the use of nonwovens. However, it is not yet known how the nonwovens behave during the concreting process, which is preventing any optimization. This is why this article aims to understand the macroscopic changes of nonwovens during concreting and identifies beneficial nonwoven properties for a waterproof bond. Seven nonwoven bonded pre-applied membranes are installed using fresh concrete and varying installation parameters and are afterward investigated using microsections, hydrostatic tests and micro-CT scans. During the installation process, the nonwovens are first compressed due to the fresh concrete load and behave like exposed to a mechanical load. Then, as vibration compaction begins, cement paste separates from the fresh concrete in a filtration process and fills the nonwoven. The fresh concrete pressure is then transferred via the cement paste so that the nonwovens can decompress due to the residual stress of the fibers. The final nonwoven thickness depends on compaction equipment, compaction time, and orientation of the nonwoven in the concrete. With the onset of compaction, the porosity within the nonwoven decreases fast so that acceptable lateral water migrations are present after 2–3 s using strong compaction equipment. The void volume of the nonwoven is decisive for the compaction time needed, so new nonwovens should be designed with low void volumes concerning a waterproof bond.

Influence of fresh concrete pressure on cover porosity investigated by single-sided proton nuclear magnetic resonance

In this study, single-sided proton nuclear magnetic resonance (1H NMR) was used to quantify the porosity of the concrete cover. Laboratory concrete samples were prepared with different formwork surfaces as well as varying fresh concrete pressures. Furthermore, to separate the influence of aggregates on porosity, hardened cement paste and mortar samples of otherwise similar composition were prepared. All samples were tested with single-sided 1H NMR, allowing depth-resolved porosity profiles of up to 18 mm. Differences in the porosity between hardened cement paste, mortar and concrete samples were primarily assigned to the formation of the interfacial transition zone. In concrete samples, the inhomogeneous distribution of the porous hardened cement paste as well as porosity and pore size variations with concrete depth appear. With increasing fresh concrete pressure, a reduction of porosity in the concrete cover can be observed, especially on surfaces made with controlled permeability formwork.

Study of formwork pressure of wall under ultra-deep vibration

In order to design vertical formwork, an accurate model to predict the lateral pressure of fresh concrete is needed in construction practice. However, there have been few studies on the lateral pressure of concrete walls subjected to ultra-deep vibration and great differences exist on the determination of form pressures and vibration depth in the current national codes. This paper reports a test of four concrete wall specimens that was carried out to study the effect of ultra-deep vibration on the lateral pressure of fresh concrete and the distribution of lateral pressure at various heights of concrete walls. Based on the vibrating liquefaction and hydrostatic pressure balance theory, a calculation model of the lateral pressure of concrete under ultra-deep vibration is put forward, together with a derived formula for simplified calculation. The predictions of the model are compared with the experimental results, and it is shown that the proposed model is able to explain the experiment observations and predict with reasonable accuracy the lateral pressure in fresh concrete walls subjected to ultra-deep vibration.

Shear strength of reinforced concrete beams with precast High-Performance Fiber-Reinforced Cementitious Composite permanent form

As a structural and constructional element, a thin-walled U-shaped precast permanent form consisting of High-Performance Fiber-Reinforced Cementitious Composites (HPFRCC) and steel wire mesh is considered. Prior to casting, transverse steel threaded bars are installed, as well as longitudinal reinforcing bars, connecting two sides of the U-shaped form for shear transfer between the precast and in-filled concrete sections and for fresh concrete pressure resistance. The present study investigated the effect of precast U-shaped HPFRCC section on the shear strength of the composite beams. As parameters for full-scale three-point bending tests, the type or materials of precast section, spacing of shear reinforcement, and shear span-to-depth ratio were considered. The test results showed that the precast U-shaped HPFRCC permanent form increased the shear contributions of in-filled concrete and shear reinforcement in the composite beam substantially. From the experimental and analytical assessment, the coefficients related to the composite action between precast HPFRCC and in-filled concrete sections, shear reinforcement along diagonal cracks, and shear span-to-depth ratio were proposed, which were applied to a previous shear strength model for the simplified shear design of proposed precast U-shaped HPFRCC-in filled concrete composite beams.

Modular construction mechanics of a European pressurized reactor steel containment liner

A European pressurized reactor (EPR) steel containment liner structure is comprised of the cylinder part and the dome part. An introduction of the steel liner structure is presented, followed by studies on the key mechanical features of the construction process using a refined finite element method. The steel liner was divided into several modules and then assembled during construction. Firstly, the equipment structure used to hoist the liner module was optimized, the lifting lug was analyzed using a multi-scale finite element model; the wind speed limit during lifting was also studied. Subsequently, the effect of internal forces during assembly between the liner modules, the lateral pressure of fresh concrete, the non-uniform temperature load, and the wind load on the cylinder module was analyzed. According to the time-varying structural performance during continuous concrete pouring and the hardening construction, an “overlapping element and birth-death element” technique was adopted to analyze the deformation and stress of the long-span steel dome liner. In addition, the stability-bearing capacities of the dome structure during construction were also studied, which took into consideration the effect of the initial geometrical imperfections and the elasto-plasticity of the material. This study presents a reference in terms of the mechanics of the construction scheme and the safety of such a type of structure.

An experimental study on the lateral pressure of fresh concrete in formwork

Most of the accidents related to concrete formwork are triggered by improper construction operations and the unsound lateral pressure estimations detailed in concrete formwork specifications. In this study a series of experiments was conducted to reveal the effect of different factors on lateral pressure. The results show that concrete slump, casting speed, and vibration mode can greatly influence the pressure; and the data obtained in the experiments differ greatly from the values yielded by different specifications. A modification is proposed to the GB50666 specification in order to improve the accuracy and reliability of pressure estimations.

고유동 고강도 콘크리트용 거푸집의 측압 특성에 관한 실험적 연구

본 연구에서는 고유동 고강도 콘크리트의 현장타설에의 적용성 검토를 위해 거푸집의 측압검토를 수행하였다. 이를 위해 타설높이 및 타설속도를 주요변수로 정하여 실험체를 제작한 후, 수행한 실험으로부터 측압 및 온도변화를 측정하였다. 고유동 고강도 콘크리트의 최대 측압은 굳지 않은 콘크리트의 측압으로 나타났으며, 타설 직후 측압이 감소하기 시작하여 12시간 이후 안정화를 보였다. 3~4시간 이후 안정화가 나타나는 보통 콘크리트와 측압이 감소하는 경향은 유사하다. 타설속도가 빠를수록 최대 측압이 크게 발생하였으며, 시간의 경과에 따른 감소량은 거의 유사하였다. 또한 수화열과 측압감소 사이에 직접적인 연관성은 나타나지 않았다. In this study, to examine the application of cast-in-place of high-flowable and high-strength concrete, an experimental study on the lateral pressure of a formwork was preformed. The experiment specimens, which have different casting height and casting speed were prepared. The lateral pressure and the change of temperature from test specimens were obtained. The maximum lateral pressure was shown to lateral pressure of fresh concrete. Immediately after placing, the lateral pressure starts to decrease and, after 12 hours, it showed a stabilization. The decreased tend of the lateral pressure was similar with normal-strength concrete, which appears stabilization after 3~4 hours from casting completion. The more casting speed is fast, the more maximum lateral pressure is high, but pressure reduction with the lapse of time was nearly similar. In addition, it was found that there was no direct relation between the hydration heat and the lateral pressure reduction.

Portable Pressure Device to Evaluate Lateral Formwork Pressure Exerted by Fresh Concrete

AbstractUse of a reliable and accurate measuring system to determine lateral pressure exerted on formwork by self-consolidating concrete can lead to economic and safe formwork systems. In this paper, a portable and field-compatible pressure device is developed to enable factors affecting lateral pressure of fresh concrete on formwork system to be explored prior to conducting onsite full-scale tests. The pressure device requires a sample of 16 L of fresh concrete to determine lateral pressure envelope that can be exerted by a concrete column of up to 13 m in height. The pressure device enables the simulation of such concrete head using an overhead pressure exerted onto the fresh concrete. A number of parameters are used to validate the pressure device, including placement rate as well as mix designs of various paste volume, slump flow, ratio of water to cementitious material, maximum size of aggregate, and dosages of chemical admixtures. The test results indicated that the repeatability of the lateral pres...

Horizontal Loading Effects of Fresh Concrete on Precast Arches

This paper investigates the horizontal effect of fresh concrete on precast arches. A number of different models of horizontal pressure of fresh concrete are considered in this regard. The effects of fresh concrete on a precast arch are represented as a ratio of maximum normal stress from horizontal action of fresh concrete to the normal stress induced by the self-weight of the precast concrete arch. A parameter study on a number of geometric and operational variables was carried out. The implications of this horizontal loading from fresh concrete are discussed within the context of the potential financial effects.

Formwork pressure induced by highly flowable concretes – material investigations and large‐scale tests

AbstractThis paper documents the results of a joint research project dealing with the loading of formwork systems used with highly flowable concretes. First calculation approaches concerning the pressure of fresh concrete on vertical formwork were developed on the basis of the investigations conducted on the material behaviour of highly flowable concretes. In doing so, construction management questions regarding, for instance, process design and quality assurance in the application of highly flowable concretes were also investigated. Different scientists from German research institutes collaborated in a research group to pool their knowledge in the field of the pressure of fresh concrete. Because of the extent and complexity of the tasks to be tackled, the joint research project was divided into five subprojects (A to E), which were performed by the research institutes according to their individual respective capacities.

Bond and Shear Behavior of Concrete Beams Containing Lightweight Synthetic Particles

Adding lightweight synthetic particles (LSP) to a concrete mixture can reduce the pumping pressure of fresh concrete, reduce the unit weight of concrete by as much as 10-20%, and improve the thermal resistance of the hardened concrete. This paper describes an experimental program that investigated the bond and shear behavior of concrete beams containing LSP. The experimental program included 27 large-scale specimens. Results indicate that the bond and shear behavior of beams with LSP additive is similar to the behavior of beams made with normalweight concrete. The results also confirm that ACI 318-08 can be used for the design of LSP concrete members for shear and the development length of steel reinforcement without the use of the reduction factor λ required for lightweight concrete. Limitations of this study and directions for future investigation are discussed.

The influence of the construction process on the deformation behaviour of diaphragm walls in soft clayey ground

AbstractConventional numerical predictions of deep excavations normally neglect the construction process of the retaining structure and choose the earth pressure at rest as initial condition at the beginning of the simulation. The presented results of simulation and measurements during the construction process of the Taipei National Enterprise Center show, that such an assumption leads to an underestimation of the horizontal wall deflection, the surface ground settlements as well as the loading of the struts in case of normally to slightly over‐consolidated clayey soil deposits. The stepwise installation process of the individual diaphragm wall panels results in a substantial modification of the lateral effective stresses in the adjacent ground. Especially the pouring process of the panel and the fresh concrete pressure causes a partial mobilization of the passive earth pressure and a distinct stress level increase in the upper half of the wall. As a consequence of the increased stresses prior to the pit excavation, up to 15% greater ground and wall movements are predicted. Moreover, the increased stress level due to the installation process of the diaphragm wall leads to substantial higher strut loadings during the excavation of the pit. Copyright © 2006 John Wiley & Sons, Ltd.

Modelling of earth and water pressure development during diaphragm wall construction in soft clay

AbstractThe influence of a diaphragm wall construction on the stress field in a soft clayey soil is investigated by the use of a three‐dimensional FE‐model of seven adjacent wall panels. The installation procedure comprises the excavation and the subsequent pouring of each panel taking into account the increasing stiffness of the placed fresh concrete. The soft clay deposit is described by a visco‐hypoplastic constitutive model considering the rheological properties and the small‐strain stiffness of the soil. The construction process considerably affects the effective earth and pore water pressures adjacent to the wall. Due to concreting, a high excess pore water pressure arises, which dissipates during the following construction steps. The earth pressure finally shows an oscillating, distinct three‐dimensional distribution along the retaining wall which depends on the installation sequence of the panels and the difference between the fresh concrete pressure and the total horizontal earth pressure at rest. In comparison to FE‐calculations adopting the earth pressure at rest as initial condition, greater wall deflections and surface ground settlements during the subsequent pit excavation can be expected, as the average stress level especially in the upper half of the wall is increased by the construction procedure of the retaining structure. Copyright © 2004 John Wiley & Sons, Ltd.

Lateral Pressure of Fresh Concrete

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